MRI is an imaging method which excites nuclear spins of an object placed in a static magnetic field and magnetically excited with a RF (radio frequency) signal having the Larmor frequency and reconstructs an image based on MR (magnetic resonance) signals generated due to the excitation.
As a morphological imaging method of the heart using MRI, a delayed enhancement (DE: Delayed Enhancement or LGE: Late Gadolinium Enhancement) imaging method is known. This delayed enhancement imaging method is an imaging method of performing imaging of an object at a time when a predetermined time has elapsed after injecting a contrast agent into the object.
In the delayed enhancement imaging for the heart, a 180-degree inversion recovery (IR: inversion recovery) pulse is applied prior to acquiring MR data for imaging, which pulse is for inverting the longitudinal magnetization Mz in the heart under the static magnetic field so that the longitudinal magnetization becomes a negative value. And, after applying the IR pulse, imaging data is acquired at a time when the longitudinal magnetization at the myocardium returns to almost zero due to the longitudinal relaxation (T1 relaxation). A period of time ranging from the application time of the IR pulse to application of an 90-degree RF pulse applied for acquiring the imaging data is called an inversion time (TI: inversion time). In other words, the TI is decided to allow the longitudinal magnetization at the myocardium to be zero, resulting in suppression of signals from the tissue of the myocardium.
Further, in the delayed enhancement imaging for the heart, in cases where a contrast agent administered into an object being diagnosed flows into its normal myocardium, the contrast agent is washed out from the myocardium approximately within a span of 10 to 15 minutes after the administration. Hence, after an elapse of 10 to 15 minutes from the administration of the constant agent, there remains only a small residual amount of the contrast agent in the normal myocardial tissue. In contrast, when the contrast agent flows into lesions such as areas of cardiac infarction, the contrast agent cannot be washed out even after an elapse of 10 to 15 minutes, and still remains in the myocardial tissue.
Meanwhile, the longitudinal relaxation time (T1) of spins in an area of cardiac infarction becomes shorter than T1 in the normal myocardial tissue due to effects of the contrast agent. Hence, when the inversion time TI is set such that imaging data is acquired after an elapse of 10 to 15 minutes from the administration of the contrast agent, the intensity of MR signals acquired from the area of cardiac infarction in which the longitudinal magnetization of spins has recovered so as to show a positive value becomes larger than the intensity of MR signals acquired from the normal myocardium. Particularly, when the TI is set to allow imaging data to be acquired at a time when the longitudinal magnetization becomes almost zero, the intensity of MR signals acquired from the normal myocardium is almost zero so that the area of cardiac infarction can be clearly depicted as higher-intensity parts.
Hence, in order to obtain a clear contrast between the normal myocardium and the area of cardiac infarction, it is essential to accurately obtain a TI that makes the myocardium have a cardiac magnetization of zero at a time when imaging data is acquired.
In view of this respect, a TI-Prep method is proposed, in which a pre-scan is performed prior to an imaging scan to acquire a plurality of frames of image data with the TI changed to different amounts, and a proper TI is obtained based on a plurality of images acquired through the pre-scan and produced in response to the different TIs. In performing the TI-Prep method, a plurality of images corresponding to the different TIs are displayed to make a user select, through a user's visual check, an image having the lowest signal intensity at the myocardium. This enables acquisition of a proper TI. That is, the TI with which the image having the lowest signal intensity at the myocardium is acquired corresponds to a proper TI.
As represented by the delayed enhancement imaging for the heart, in imaging using the IR method, it is essential to set the TI with precision. In particular, in the IR imaging performed with the use of the absolute-value image signals, optimally setting the TI is an important issue, because signals are sampled discretely but a minimum value of signals is theoretically at one point. It is further desired to properly set the TI in imaging in order for obtaining more useful diagnosis aid information.
An object of the present disclosure is to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which can obtain useful diagnostic information from portions being imaged, including the heart, by imaging based on an IR method which uses a properly set TI.